Lyotropic phase transitions in phospholipids as evidenced by small-angle synchrotron X-ray scattering

Hydration is an important factor in regulating the phase behaviour of lipid s and besides affects their interactions with other compounds relevant for biological membranes. We present a reliable and fast method to detect and c haracterise hydration-induced phase transitions in phospholipids by means o f small-angle synchrotron X-ray scattering. Films consisting of aggregation s of representatives of the two important lipid classes lecithins (DppC (a) , pOpC and OppC,(a) for abbreviations, see below) and cephalins (DPPE and D OPE) were investigated at room temperature in dependence on relative humidi ty. Qualitative changes in the sets of the diffraction patterns obtained in dynamic hydration/dehydration scans were taken as markers indicating the e xistence of lyotropic phase transitions. The efficiency of this methodology is demonstrated by illustrating the course of hydration-driven phase trans itions between lamellar as well as nonlamellar phases. In detail, this was realised for chain melting in the mixed-chain lipids, POPC and OPPC, and fo r a novel nonlamellar-phase transition for DOPE between a disordered invert ed ribbon phase designated as P. and the canonical H-II phase, respectively .